The actual term "cardsharing" has many names like CS (shouldn't be confused with a popular game called the Counter Strike), card shares, control word sharing, and so on, but they are commonly used to point out the method of using a smart card (or multiple cards) over a network. Basically a legitimate satellite smart card can be inserted into a receiver's card slot (host), and then another receiver (client) which could be located either in the next room or a thousand miles away will have an access to the card via internet or local network.

The "client" device will have a full access to all TV channels which are available through the physical card plugged into the "host" receiver, thus the name "satellite cardsharing". So the actual sharing simply means an access to a remote card from a device which doesn't have it plugged in physically.

To get things rolling you need to obtain network-capable equipment like the Linux-based Dreambox receiver. These types of receivers have an Ethernet connection just like computers do, and are able to use specially developed software and plugins to send and receive codes, aka information. The software includes such names like Newcs, Mgcamd, Newcamd, CCcam, and others that have gained wide popularity for being able to allow satellite TV cardsharing.

In order for receivers to be able to communicate over a network they've got to be properly configured. First of all, a device needs to know where the card is situated. The device with a card is basically your satellite cardsharing server, or a host. Those receivers without the smart card could be called satellite cardsharing clients. Common computer-based IP address is used to pinpoint an exact location of multiple devices so that they could communicate between each other, so device #1 knows where device #2 is located by its direct IP number.
For example, you've got yourself a satellite card sharing server which is called "CS" on a device called "Host". So your "Host" is running satellite cardsharing software and "shares" (aka hosts) a satellite smart card in its slot. Your "CS" server "speaks" via CCcam protocol, meaning that it can communicate with other receivers which are "speaking" the same language. Now, to cut the complicated stuff off, there could be literally hundreds of receivers, and in order for them to "share" the content between each other and through the satellite TV cardsharing server they all need to be properly configured.
Basically, in modern cardsharing it is about paying for access to a "share", or you could split the cost of a subscription between your friends, but no matter what you'l do, it is a very convenient and practical way of saving your money & pay less and get much more unlike with the conventional satellite provider.